As explained in U.S. Pat. No. 4,517,981, the teachings of which are incorporated herein by reference in its entirety, each joint of the human foot has a specific function and covers a certain range of motion. Human feet may be categorized into various types (i.e., structures and shapes). A human foot type effects the impact endured by a foot during heel strike (e.g., while walking, jogging, or running). A human foot type may be determined based on the shape of the foot, the shape of the arch of the foot, and the extent of rotational movement of the foot (i.e., pronation).
A neutrally functioning foot ideally contacts the ground in a slightly supinated or high-arched position and pronates (i.e., flattens out) to conform to the ground. In contrast, some neutral, flat and high-arched feet may already be pronated (i.e., flat) at heel strike (or foot contact) and, as such, cannot conform to the ground (decelerate) and absorb shock.
In a high-arched foot, the first metatarsal bone is generally lower than the other metatarsal bones, in relation to the plane of the rearfoot or undersurface of the heel. This condition is commonly referred to as a plantar-flexed first metatarsal.
In a flat foot, the first metatarsal bone is usually elevated in relation to the plane of the rearfoot or undersurface of the heel. The first metatarsal bone may be in the same plane as the remaining metatarsal bones or at higher or lower planes. The first metatarsal bone in a flat foot can be very flexible and can bend easily when weight is placed on the foot to allow the rearfoot to flatten out or pronate with no resistance.
Standing extended periods of time or walking long distances can be uncomfortable if the feet are not in functionally corrected positions. Many of today's footwear, even walking and running shoes and orthotic inserts, can attempt to address high-arched or flat feet, but without addressing other interrelated issues in the very complex bone, ligaments, and muscle structures in the feet and lower extremities, these attempts can be ineffective at best or exacerbate otherwise subtle discomforts at worst.